What year was a law passed which stated that thereafter all new designs for coins and currency would bear the inscription In God We Trust?

The following explanation is provided by the United States Mint:

"Legislation approved July 11, 1955, made the appearance of 'In God We Trust' mandatory on all coins and paper currency of the United States. By Act of July 30, 1956, 'In God We Trust' became the national motto of the United States."